The public switched telephone network (PSTN) is the network of the public circuit-switched telephone networks that provide the majority of voice services. The PSTN is largely governed by technical standards and uses standardized addresses, namely telephone numbers. PSTN was originally a network of fixed-line analog telephone systems; however the present PSTN is almost entirely digital, and includes mobile phones. While the technology continues to change, the majority of users in the public still use the typical fixed telephone handset service which is sometimes referred to as the Plain Old Telephone Service (POTS).
In a traditional public switch telephone network there are two basic system elements, namely the switching device and the access device. The switching device in a local exchange is generally referred to as a Class 5 switch. The class 5 office is the local exchange or end office, and it delivers dial tone to the customer. The end office, also called a branch exchange, is the closest connection to the end customer. The function of a Class 5 switch in rural areas is often performed by some form of remote switch or Remote Digital Terminal (RDT) which is installed at the original switch site to handle local switching. The Class 5 switching infrastructure is then physically located in a larger population center. The switch provides the call control, call features and routing information necessary to complete a telephone call within a carrier's exchange. The term switch as used herein refers to any of the various switch types.
The access device, sometimes referred to as a Digital Loop Carrier (DLC) or Broadband Loop Carrier (BLC), provides the physical interface to the subscriber's telephone equipment. This interface typically includes battery voltages to power the telephone equipment, ringing voltages necessary to ring the telephone equipment when an incoming call in received, and the analog to digital conversion required to interface with the switches. An access device can encompass IP Media Gateways (IPMGs) when capable of communicating by internet protocol. An IPMG is any type of access device that can support IP ESA provisioning, supporting multicasting.
Under normal operation, the access device or IPMG and the switch are required to have a reliable communications path available for a telephone call to be made, even between two subscribers connected to the same access device or IPMG. Despite the size of the public switched telephone network, it averages approximately 99.999% availability and employs various redundant features, especially in the older established networks and those with dense populations.
Digital Loop Carriers (DLCs) or next-generation access systems such as BLCs are excellent devices for terminating subscriber access lines, reducing loop plant or providing concentration of access lines. However, most DLCs also represent a critical point of failure in a carrier's network because if the link between the host switch and the DLC is severed, the DLC will be unable to complete any local calls including emergency 911 calls. Today, over half of all access lines in the United States are terminated on DLCs, few of which have any back up or fail-safe switching capability. This represents a large and potentially critical point of failure in the PSTN and a public safety concern for all carriers.
One area of concern for the public is network redundancy to achieve Emergency Stand Alone, especially with the high cost of added equipment typically required for such redundant capability. The redundancy for upgrades to replace older networks and for new installations drives the cost of network construction up significantly. Rural telephone market consists of large geographic areas with relatively low density of subscribers. The ability to economically build a fully redundant network is not feasible.
Emergency Stand Alone also called Emergency Standalone (ESA) refers to the capability of an area within a voice network to be self sufficient and capable to operate and complete all calls originating and terminating within that service area even if the connection to the broader public switched telephone network has failed. There are typically two levels of calls that are handled in the ESA mode, namely the regular calls that would normally be routed via the switch and emergency calls such as 911.
By way of example, the loss to the switch could be the result of equipment failure, a fiber optic failure or a metallic cable failure. It could also be related to a power outage in which the emergency and/or battery power fails. Also, internal operational/software failure can contribute to the loss. Typically, every access device in the network which could suffer such connectivity loss would be required to have an ESA device in order to ensure calling services would be intact in the event of a hardware or fiber failure.
Historically, ESA service has been provided by a central office host switch or a subtended remote switch unit, both of which are broadly referred to herein as switches. If the connection between the central office host switch and the broader public switched telephone network was lost, the central office host switch would attempt to complete all local calls and route all 911 calls to a local public service answering point. Similarly, if the connection between the remote switch unit and the central office host switch was severed, then the remote switch unit would continue to attempt all local calls within its service area and generally route all 911 calls to a pre-designated emergency responder, such as the local fire or police department. Access concentration devices such as Digital Loop Carriers (DLCs) and the newer generation of Broadband Loop Carriers (BLCs) traditionally do not have any switching capability and thus lack emergency stand alone service.
In a general sense, ESA provides a mechanism whereby the access device can still provide the ability to complete calls between subscribers attached to it even in the event connectivity to the switch is lost. As noted, this typically requires that additional hardware is required and that the ESA domain, or scope of the subscriber lines covered by the ESA, is limited to a predefined area.
There have been some attempts for providing emergency stand alone services such as by installing low end switches in remote locations or requiring a switch for every town, and some networks even required all central offices to have switches. Large remote sites sometimes have installed cards that would allow form of stand alone operation.
As the technology has evolved, the handling capacity has increased and the equipment size has decreased. In combination with the enhancements for soft switching, there has been a trend towards consolidation and larger central offices as opposed to many discrete offices. Soft switching refers to the processing within the switch controller wherein the switch maintains a database of the endpoints and their respective addresses. There are tables that enable the switch to route traffic to the intended destination as the switch has predetermined information about the endpoint. Such trends create a greater vulnerability to loss of voice lines as a single failure could lead to greater outages of voice communication.
As Voice over Internet Protocol (VoIP) telephone service has gained market momentum in the PSTN market, the requirement for ESA has not diminished. VoIP is a technology that allows a user to make voice calls using a broadband Internet connection instead of a regular (or analog) phone line. Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband generally refers to the routing of voice communications over the Internet or through any other IP-based network. VoIP systems traditionally employ two components, namely a switch and the terminal adapters or client. The switch is the brain that determines the processing for the various endpoints, wherein the endpoints are subservient to the switch.
VoIP providers follow established protocols to carry voice signals over the IP network, wherein the system converts voice into digital signals that travel over the Internet. If calling a POTS handset, the signal is converted to a regular telephone signal before it reaches the destination. VoIP allows calls directly from a computer, a special VoIP phone, or a traditional phone connected to a special adapter. In addition, wireless hot spots in locations such as airports, parks, and cafes allow you to connect to the Internet and may enable VoIP service wirelessly.
There are a variety of VoIP protocols as well as VoIP systems. There is typically a terminal adapter that is coupled to the hand set during the dialing of a telephone call that sends the dialing information back to a switch and the switch determines the outcome such as converting the dialed numbers into the proper format and providing some ring tones back to the user. The IPMG takes user input and converts between the VoIP and handset. Redundancy typically requires that there is always a switch between the VoIP client and the switch. In dense areas, there may be multiple connections to a switch such that the VoIP clients can be re-routed to the switch if one connection is inoperable. However, in rural environments, there is no guarantee of a surviving path between the VoIP clients and the switch controlling the phone service if a connection is broken.
The traditional approach to provide ESA capabilities in the VoIP scenario is not very different from the approach taken in the division multiplexing (TDM) environment. Typical VoIP based ESA requires that for each ESA domain an additional device or module would need to be installed to guard against connectivity loss to the switch.
As can be seen in FIG. 1, a prior art VoIP system 5 is depicted with a switch that can be a central office host switch or a subtended remote switch unit, which includes Class 5 switches and RDT.
The switch 10 may be coupled to other switches (not shown) or a central host to provide some form of ring network. In this system 5, there is a first DLC A 20 coupled to a first POTS handset 25. The DLC can be coupled to multiple handsets. The DLC is basically an analog/digital (A/D) converter that performs the necessary translation processing between the POTS handsets and VoIP infrastructure, and establishes the various communications protocol with the switch 10. The switch 10 in this example is also coupled to a second DLC 30 as well as nested access devices 40, 50. The third DLC 40 is also coupled to a POTS handset 55. It should be understood that this is just one representative example of the prior art implementations and other variations are known to those skilled in the art.
By way of an illustrative example, if a party using a POTS handset 25 dials a telephone number, the DLC 20 performs the processing to convert the Dual-tone multi-frequency (DTMF) or pulse telephone number into the proper VoIP communications protocol. The switch 10, with its apriori knowledge of the telephone numbers in the network, routes the signal to the proper location and corresponding handset 55 whether it is a wireless caller, VoIP client or another POTS handset. If communicating to another POTS handset one or more receiver DLCs 30, 40, 50 is used to translate the digital VoIP communications back into the POTS protocol so that it is received by the receiver POTS handset 55 in the proper format.
The switch 10 typically follows call admission control that manages administrative aspects such as whether the dialed telephone number is valid, whether the caller is allowed to make the call based on the caller account, whether the receiving party is in the network, or whether the receiving party is busy or on a failed network. Various handshaking occurs prior to the connection between the caller and receiver which is administered by the switch prior to successful completion of the call.
In the event that the switch is inoperative, ESA service in the state of the art requires pre-existing added hardware elements. In order to provide such full ESA capabilities, an ESA device or switch (not shown) must be added and coupled to DLC A-D 20, 30, 40, 50. Furthermore, in the event that the link from the switch 10 to DLC B 30 were to fail, the connectivity between DLC C 40 subscribers and DLC D 50 subscribers would require complex routing knowledge provisioned in each ESA device to allow calls to be completed properly.
For example, when a user wants to make a telephone call from a standard POTS handset on a network that employs VoIP, the user picks up the receiver and a dial tone is provided by the VoIP client. The user dials the desired telephone number and The VoIP client makes a number of efforts to route the call to the switch.
Several approaches and systems have been contemplated to provide redundancy with limited success. There are collapsed rings providing local redundancy but the lines are typically in the same trench and this would not aid in a failure further upstream from the ring.
Some existing systems employed an auxiliary switch that waits for the master switch to be offline and then takes control of the network traffic. This is not a distributed approach and requires a sufficient number of auxiliary switches which can be costly to implement. One embodiment of the invention is a distributed ESA approach that adds the multicast functionality into the VoIP endpoints such that they can operate independently if the switch is offline.
In addition, there are a few implementations that have evolved for converting the VoIP to POTS electrical interface. For example, in the Vonage system, a user sets up an account on the user's computer. Vonage wants a switch with the database of known endpoints that maps phone numbers to IP addresses. The Vonage system establishes an electronic adapter in every house and converts the VoIP to the POTS handset at the house. A disadvantage of this approach is that a loss of power will result in loss of the telephony.
The telephone company system approach places the VoIP-POTS transfer mechanism as part of the network and customer is presented the standard two-wire POTS connection at the house. If the power is lost at the house, the phone line may still be powered as long as there is power to the VoIP transfer mechanism, which typically has some battery backup.
In general, the industry move towards digital communications and VOIP has facilitated the migration away from remote offices which increases the exposure to loss of emergency voice lines in the event of a failure at a central office or central office line.
The fiber-to-the-home (FTTH) initiative has been slowly converting existing cables to fiber but the economics requires a certain density to make the conversion cost effective. Present rural implementations use fiber out to a remote cabinet and the ‘last mile’ to the home using existing copper wire using, for example, ADSL, or in-the-house POTS adaptation. While new developments can go immediately to fiber, it will be many years before all the cabling is replaced by fiber, especially in less dense areas.
The premise devices are also continuing to evolve and incorporate processing and communications capability. The premise device can be a network interface device (NID) also sometimes referred to as a network interface unit (NIU) which is a multi-functional device that acts as an interface providing code conversion, protocol conversion and buffering used for communications to/from a network. The NID in some applications allow a number of independent devices, with varying protocols, to communicate with each other and converts a device protocol into a common transmission protocol. The transmission protocol may be chosen to accommodate directly a number of the devices used within the network without the need for protocol conversion for those devices by the NID.
Despite the advances to a purely digital network, however, as long there are standard handsets in the field employing the POTS interface, there is a requirement for converting VOIP to POTS.
Thus there is a need for emergency stand alone services since the network is not resilient enough to tolerate failure without disrupting life line voice services. One of the principal economic challenges to providing full ESA capabilities is the requirement for added equipment. In addition, there is the additional complexity of programming routing information needed to complete calls under the existing ESA even though the ESA domain is static and refers to a single physical location. What is needed, therefore, is a system and techniques for routing ESA calls without the burden and cost of auxiliary hardware such as switches.